Process for the production of nitriles

ABSTRACT

An improved process is provided for the production of nitriles from hydrocarbons by reaction with an oxygen-containing gas comprising oxygen, air or a gas enriched in oxygen relative to air and ammonia in the presence of a suitable catalyst. In the process, a selective separator provides recycle of a substantial portion of the unreacted hydrocarbon as well as for a controlled amount of a gaseous flame suppressor in the system. The gaseous flame suppressor comprises a substantially unreactive hydrocarbon containing 1 to 5 carbon atoms, carbon dioxide and nitrogen when present in the feed to the ammoxidation reactor. The use of air or oxygen-enriched air in the feed to the ammoxidation reactor is particularly advantageous from an economic view in combination with a pressure swing adsorption unit as the selective separator. The process is characterized by high selectivity to the formation of the product.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 178,117, filed Apr. 6, 1988, now abandoned, which, in turn, isa continuation-in-part of U.S. patent application Ser. No. 124,731,filed Nov. 24, 1987, now abandoned.

FIELD OF THE INVENTION

The present invention is directed to a process for producing nitrilesfrom hydrocarbons, an oxygen-containing gas and ammonia in the presenceof a suitable catalyst under conditions which achieve high efficiencyand selectivity toward the desired product.

BACKGROUND OF THE INVENTION

The production of nitriles by ammoxidation of an appropriate hydrocarbonin the presence of a suitable catalyst is well known. The production ofacrylonitrile, for example, from a gaseous feed of propylene, ammoniaand air is described by Bruce E. Gates et al in Chemistry of CatalyticProcesses, McGraw-Hill (1979), pp. 380-384.

The feed is sent to an ammoxidation reactor where, in the presence of asuitable catalyst, acrylonitrile is produced along with lesser amountsof other nitrogen containing compounds. The effluent from theammoxidation reaction is quenched with water and the desired productsare obtained in the liquid phase. The gas phase byproducts, typicallyoxygen, carbon dioxide, carbon monoxide and unreacted hydrocarbon, arecombined with natural gas and sent to a boiler for combustion asdisclosed, for example, in Yoshino et al., U.S. Pat. No. 3,591,620 andCallahan et al., U.S. Pat. No. 4,335,056.

More recently, Khoobiar et al., in U.S. Pat. No. 4,609,502 disclosed acyclic process for producing acrylonitrile using propane as a startingmaterial which is initially dehydrogenated catalytically in the presenceof steam to form propylene. This is in contrast to most conventionaldehydrogenation processes which avoid steam primarily due to the costsinvolved. After ammoxidation, the effluent is quenched, the desiredproduct removed, and the off-gases, including propylene and propane, aresent to an oxidation reactor to remove oxygen by selective reaction withhydrogen to form water vapor. The gas mixture exiting the selectiveoxidation reactor includes substantial amounts of methane, ethane andethylene, which are by-products of dehydrogenation, and unreactedpropylene in addition to carbon oxides. As an option, this gas mixtureis split and a portion is sent to a separator which removes only carbondioxide. A portion of the effluent from the separation is purged toremove light hydrocarbons. The nonpurged stream is mixed with theremainder of the oxidator effluent, fresh propane and steam, ifnecessary, and sent to the dehydrogenator where the propane is convertedto propylene. Another option is to cool and liquify the C₃ hydrocarbonstherefrom and then vaporize them prior to recycle.

The aforementioned process suffers from several disadvantages. Theamount of propane which is converted to propylene is only in the rangeof about 20 percent to 60 percent per pass, typically about 40 percent,and therefore, about 60 percent of the propane feed is recycledthroughout the system. At conventional velocities, the presence of suchlarge amounts of propane along with hydrogen and other gases can producehigher pressures in the ammoxidation reaction zone which can, in turn,result in decreased yields of acrylonitrile. This problem could beovercome by using a more efficient dehydrogenation catalyst, if suchwere commercially available. Also, there is no practical way in thisscheme to selectively remove by-products of propane dehydrogenation,such as methane, ethane, ethylene and the like, thereby preventing theiraccumulation in the system. Providing a purge stream to remove thesegases will also cause removal of some of the circulating propane andpropylene. As the process is being carried on in a continuous manner,this loss of starting material causes a significant decrease in theyield of propylene. It is disclosed that propane and propylene arerecovered from the purge stream prior to venting. Additionalrefrigeration is therefore necessary to liquify the propane andpropylene. This apparatus, as well as that required to vaporize themprior to recycle, significantly adds to the capital cost of the process.

Another disadvantage of the Khoobiar et al process stems from the use ofthe selective oxidation reactor to treat the gaseous effluent from thequencher. The gases exiting the quencher are at ambient temperature andmust be heated prior to introduction into the oxidation reactor in orderto promote oxygen removal. Because there is a significant amount ofoxygen in the quencher effluent, the heat of reaction generated in theoxidation reactor can result in excessive temperatures in the system.There are three options to alleviate this problem. First, the amount ofoxygen entering the oxidation reactor can be reduced by other means.Second, multiple reactors can be utilized with a cooling means betweeneach pair of reactors. Third, a portion of the effluent from the reactoris passed through a cooling means and recycled to the feed to reduce theinternal temperature of the reactor. None of these measures isattractive from the viewpoint of cost and efficienty.

The oxidation reactor in the Khoobiar et al process is operated withoxidation catalysts such as noble metals (e.g., platinum). Olefins andcarbon monoxide, which are generated in the dehydrogenation reactor, areknown to poison these catalysts, as disclosed in Catalytic Processes andProven Catalysts, Charles L. Thomas, Academic Press (1970) pp. 118-119.Accordingly, multiple oxidation reactors must be used to allow forfrequent regeneration of the catalyst which represents yet anotheraddition to production costs (U.S. Pat. No. 4,609,502, column 4, lines51-56).

It is therefore apparent that industry is still searching for a costeffective process of converting hydrocarbons into nitriles oranhydrides. Applicants have discovered a process which is cost effectiveand in which the disadvantages of the aforementioned systems aresubstantially reduced or eliminated. Moreover, in comparison toconventional processes, the thermal requirements of Applicants' processare markedly reduced.

SUMMARY OF THE INVENTION

A process is disclosed for the production of nitriles comprisingreacting in a suitable reactor a hydrocarbon, an oxygen-containing gas,preferably oxygen-enriched air, and ammonia gas in the presence of asuitable catalyst under operating conditions which produce the desiredproduct at low conversion and high selectivity. The product stream isquenched with a liquid to form a liquid phase containing the desiredproduct and a gas phase which is introduced into a suitable selectiveseparator, preferably a pressure swing adsorption unit, to separatesubstantially all of the unreacted hydrocarbon which is recycled to thereactor. The process provides an effective amount of a gaseous flamesuppressor, i.e. from about 30 to about 95 percent by volume, in the gasstream which is introduced into the selective separator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates in a block diagram a present conventional process ofproducing acrylonitrile.

FIG. 2 illustrates in a block diagram a prior art process of producingacrylonitrile utilizing a recycle step.

FIG. 3 illustrates in a block diagram an embodiment of a process forproducing acrylonitrile in accordance with the present inventionutilizing air or oxygen-enriched air as a feed to the ammoxidationreactor, a pressure swing adsorption unit as the separator wherein theflame suppressor contains a major portion of nitrogen.

FIG. 4 illustrates in a block diagram an embodiment of a process ofproducing acrylonitrile in accordance with this invention wherein theflame suppressor contains a major portion of propane.

FIG. 5 illustrates in a block diagram an embodiment of a process ofproducing acrylonitrile in accordance with this invention wherein theflame suppressor contains a major portion of carbon dioxide.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the process of this invention, a hydrocarbon is reacted with anoxygen containing gas comprising pure oxygen, air or a gas enriched inoxygen relative to air in the presence of an ammoxidation catalyst.Suitable ammoxidation catalysts are those catalyst that will catalyzethe production of the desired nitrile under the conditions utilized inthe reactor. These catalysts and their use are conventional and wellknown to one of ordinary skill in the art.

Illustrative of products, and their respective starting materials, whichcan be advantageously produced by the method of this invention areacrylonitrile from propylene, methacrylonitrile from isobutylene,phthalonitrile from o-xylene and the like. In the interest of brevity,the subject process will be described with reference to the productionof acrylonitrile, but is in no way intended to be limited thereto.

Turning to the drawings, a process currently utilized commercially toproduce acrylonitrile is illustrated in FIG. 1. Propylene, ammonia andair are fed into a conventional reactor containing a suitableammoxidation catalyst. The reactor may be of any conventional fixed orfluidized bed design, typically the latter. Such processes, which do notinvolve a recycle step, can utilize air or oxygen-enriched air in thereactor feed, although air is normally used for reasons of economy. Theoxygen concentration in the reactor feed is not considered critical withregard to the accumulation of other gases, primarily nitrogen, in thesystem due to the lack of recycle. Those skilled in the art are awarethat the oxygen content in the feed in such a process must be regulatedin regard to other aspects of the process.

The reactor product gases are cooled in a heat exchanger, not shown, toform steam and then passed to a water quench column or tower to dissolvethe products, i.e. acrylonitrile, acetonitrile, acrolein and hydrogencyanide as well as unreacted ammonia. The acrylonitrile is subsequentlyrecovered from the aqueous solution by conventional methods. Theoff-gases from the quench tower are combined with natural gas andcombusted in a boiler to generate steam. The off-gases of the boiler arevented. Since there is no recycle provided in such a process, the yieldof acrylonitrile realized is directly related to the efficiency of thereactor.

FIG. 2 illustrates the cyclic process for producing acrylonitriledisclosed in Khoobiar et al U.S. Pat. No. 4,609,502. In this process,propane and steam are fed into a dehydrogenator to form propylene whichis then mixed with oxygen and ammonia and fed into an ammoxidationreactor such as described in FIG. 1. The product is fed to an aqueousquench tower as in FIG. 1 and the products withdrawn in solution. Thegaseous take-off from the quench tower, typically containing oxygen,hydrogen, carbon monoxide, carbon dioxide, methane, ethane, ethylene,propane and propylene, is fed to a selective oxidation reactor. Aspreviously indicated, it is generally essential for the efficientoperation of such a reactor to heat the gas mixture prior tointroduction therein.

A portion of the off-gas from the oxidation reactor is passed to aseparator to remove carbon oxides by an undisclosed mechanism. A portionof the separator effluent, which contains light hydrocarbons andhydrogen, is purged, treated to remove propane and propylene anddiscarded, thereby preventing build-up of by-products in the system. Thepropane and propylene are combined with the remainder of the separatorand oxidator effluents and recycled to the dehydrogenator. It is, ofcourse, necessary for the oxidator to be effective in removing alloxygen from the quench tower effluent to prevent significant loss ofeffectiveness of the dehydrogenator. It is also necessary for the oxygenfeed to be pure oxygen since the use of air or oxygen-enriched air wouldproduce a rapid accumulation of nitrogen in the system. This would, inturn, require the purging of a larger portion of the recycle stream withresulting loss of efficiency.

In contrast to the processes illustrated in FIGS. 1 and 2, the processof the present invention provides a selective separator, preferably apressure swing adsorption unit, which effectively removes a substantialportion of the reactant hydrocarbon so that it can be recycled to theammoxidation reactor, thus providing high propylene conversion andeconomy of operation. As utilized herein, the expression "a substantialportion" as it pertains to recycle of the reactant hydrocarbon, means atleast 80 percent by volume where the oxygen-containing gas in the feedto the ammoxidation reactor is air, at least 90 percent by volume wherea mixture of equal parts of pure oxygen and air is utilized as theoxygen-containing gas and at least 95 percent by volume where theoxygen-containing gas is pure oxygen. In each instance, these areminimum percents. Utilizing pure oxygen as the oxygen-containing gas,for example, the amount of reactant hydrocarbon recycled is typically97-99 percent by volume.

As the selective separator contemplated herein typically requires apressurized feed, the process of the present invention provides aneffective amount of a gaseous flame suppressor to minimize theflammability potential at the introduction of the gaseous effluent fromthe quench tower into the means to raise the pressure thereof, e.g. acompressor. The gaseous flame suppressor of the present inventioncomprises a substantially unreactive hydrocarbon having from 1 to 5carbon atoms, carbon dioxide and, where air or enriched air is utilizedas the oxygen-containing gas in the feed to the ammoxidation reactor,nitrogen. By "substantially unreactive" is meant that at least 95percent by volume of the hydrocarbon component of the gaseous flamesuppressor will pass through the ammoxidation reactor unchanged. Thesubstantially unreactive hydrocarbon can be a saturated or unsaturatedhydrocarbon with saturated being preferred.

The amount of the gaseous flame suppressor mixture in the subject cyclicprocess is controlled so that it will be maximized at the point orpoints in the system where it is of greatest advantage, i.e. the feed tothe pressure raising means. It will be appreciated that, when pureoxygen is utilized as the oxygen-containing gas in the feed to theammoxidation reactor, nitrogen will not be a component in the gaseousflame suppressor. In embodiments of the subject process to be describedherein, each of the components of the gaseous flame suppressor mixtureof the present invention will comprise a major portion thereof.

Examples of specific substantially unreactive hydrocarbons utilized as acomponent of the subject gaseous flame suppressor and the reaction inwhich they are utilized include: propane for the production ofacrylonitrile from propylene; n-butane for the production ofmethacrylonitrile from iso-butylene; and the like.

Unreacted hydrocarbon reactant, e.g. propylene, butylene and the like,while not a major portion of the gaseous flame suppressor, appears toenhance the flame suppressant capacity thereof. The amount of unreactedreactant hydrocarbon present in the gaseous flame suppressor mixturewill depend on the percent per-pass conversion of the reactanthydrocarbon entering the ammoxidation reactor which is converted toproducts. Those skilled in the art will appreciate that factors such aschoice of catalyst, operating pressures and the like can be adjusted tohave the ammoxidation reactor operate at a desired conversion of thereactant hydrocarbon in the feed thereto. At lower operatingconversions, e.g. 60 percent conversion, there will be a greater amountof unreacted reactant hydrocarbon circulating in the system. Forexample, using substantially pure oxygen in an ammoxidation reactoroperating at 80 percent conversion in accordance with the invention,there will be only about 10 to 15 percent unreacted reactant hydrocarbonin the quench tower effluent whereas, at 60 percent conversion, therewill be a greater percent unreacted reactant hydrocarbon in the quenchtower effluent. The composition of the gaseous flame suppressor will beadjusted accordingly.

The process of this invention is advantageous in that it provides theefficiency of recycle afforded by the process illustrated in FIG. 2, yetis free of the disadvantages inherent therein while, at the same time,being less complex. The advantages of the present invention are, inlarge measure, provided by the incorporation into the process of aselective separator which retains a substantial portion of the unreactedreactant hydrocarbon in the system. The gaseous flame suppressor of thesubject process may be partially or totally recycled, depending on theoperating conditions and feed to the system. In a preferred embodiment,the separator is a pressure swing adsorption unit.

Pressure swing adsorption (PSA) is a well known process for separatingthe components of a mixture of gases by virtue of the difference in thedegree of adsorption among them on a particulate adsorbent retained in astationary bed. Typically, two or more such beds are operated in acyclic process comprising adsorption under relatively high pressure anddesorption or bed regeneration under relatively low pressure or vacuum.The desired component or components may be obtained during either ofthese stages. The cycle may contain other steps in addition to thefundamental steps of adsorption and regeneration, and it is commonplaceto have two or more adsorbent beds cycled 180° out of phase to assure apseudo continuous flow of desired product. While it is conventional forthe adsorption step of a PSA cycle to be carried out under pressure, itcan run at ambient pressure with desorption under vacuum. It is thedifference in pressure between the adsorption and desorption stageswhich is essential for operation of the system.

Referring to FIG. 3 and a process for producing acrylonitrile, a feedinto a conventional ammoxidation reactor comprises propylene, ammoniaand air or oxygen-enriched air. In accordance with this invention,oxygen-enriched air preferably contains from about 30 to about 80, mostpreferably from about 55 to 65, percent by volume of oxygen. Suchmixtures may be produced by adjusting the capacity of a conventionaloxygen-producing unit, e.g. a conventional PSA unit, or by mixing pureoxygen with air in the proper proportions. The use of oxygen-enrichedair will produce a minimum concentration of nitrogen in the feed to theammoxidation/oxidation reactor of from about 10 to 20 percent by volumeof the feed into the reactor depending on the operating conversion ofthe reactor as discussed above. The minimum concentration of nitrogen inthe reactor feed will result in the desired level of nitrogen in thefeed to the compressor for the PSA unit separator as discussedhereafter.

The ammoxidation reactor utilized in the present process is conventionaland may employ either a fixed or fluidized catalyst bed. A typicalexample of an ammoxidation reactor is disclosed in Angstadt et al., U.S.Pat. No. 4,070,393 and Gates et al., ibid, pp. 381-383, eachincorporated herein by reference. The reactor contains a conventionalammoxidation catalyst, such as bismuth-molybdenum oxide, iron-antimonyoxide, uranium-antimony oxide precipitated on silica and the like. Othersuitable catalysts are disclosed, for example, in Chemistry of CatalyticProcesses, Gates et al, McGraw Hill (1979) pp 349-350, and Yoshino etal, U.S. Pat. No. 3,591,620, incorporated herein by reference.Additional suitable catalysts are known to those skilled in the art.

The ammoxidation reaction is conducted at a temperature of from about375° to 550° C., preferably from about 400° to 500° C., at lowpressures, typically in the range of from about 2 to 30 psig, preferablyfrom about 3 to 20 psig. The reactants are passed through the reactor ata relatively low velocity, typically in the range of from about 1.75 to2.2 ft./sec. The ratio of oxygen to propylene in the feed is suitably inthe range of 1.6:1 to 2.4:1 by volume. The ratio of ammonia to propylenein the feed is suitably in the range of 0.7 to 1.2:1 by volume.

The ammoxidation reaction results in the production of a major amount ofacrylonitrile and minor amounts of acrolein, hydrogen cyanide,acetonitrile, and carbon oxides as well as unreacted oxygen, propyleneand nitrogen. Commercially available propylene typically contains asmall amount of propane unless additional costly processing isundertaken to remove it. This is advantageous since propane exerts apositive effect on the gaseous flame suppressor of the present process.In this embodiment, the propane is substantially recycled from the PSAunit. This gaseous mixture is quenched or scrubbed with a liquid, suchas water, to dissolve the water soluble compounds for subsequentseparation and recovery of acrylonitrile, acetonitrile and hydrogencyanide.

The gas phase effluent from the quench tower is introduced into a PSAunit separator wherein unreacted propylene is separated from the othergases in the mixture. Although the adsorption stage of the PSA unit canbe at ambient pressure, it is preferred to pass the quench tower gaseouseffluent through a compressor or other suitable means to increase thepressure thereof prior to introduction into the PSA unit. Experience hasshown that the flammability potential in the system is greatest in thecompressor. Therefore, the volume percent of gaseous flame suppressor isat its maximum in the quench tower gaseous phase, i.e. the feed into thecompressor, to minimize the flammability potential. The operationconditions of the system are adjusted to provide for this.

The gas phase effluent from the quench tower preferably contains, on avolume basis from about 1 to 3 percent of carbon monoxide, from about 3to 5 percent of propylene, from about 6 to 8 percent of oxygen, fromabout 10 to 15 percent of carbon dioxide, and from about 50 to 70percent of nitrogen and from about 10 to 20 percent of propane, thelatter three comprising the gaseous flame suppressor. It must be bornein mind that this example is given relative to the synthesis ofacrylonitrile from propylene utilizing oxygen-enriched air containingapproximately 60 percent by volume of oxygen as the oxygen-containingfeed to the ammoxidation reactor which is operating at 60 percentconversion of the propylene. More preferably, the volume percent ofpropylene, nitrogen and carbon monoxide in the quench tower effluent isfrom about 35 to 70 percent by volume. In the process illustrated inFIG. 3, the preferred selective separator is a PSA unit operating underpressure provided by a compressor. The compressor increases the pressureof the quench tower gaseous phase to the preferred operating pressure ofa PSA unit, typically in the range of from about 3 to 50 psig,preferably from about 20 to 40 psig. The range of preferred operatingpressure may vary to an extent depending on the adsorbent utilized.

The adsorbent in the PSA unit may be any art recognized material whichadsorbs propylene to a substantially greater degree than propane orcarbon dioxide. Silica gel is a preferred adsorbent material.

By proper selection of the adsorbent in the PSA unit, the operationthereof can readily be controlled utilizing art-recognized manipulationsso that the recycle stream formed therein contains a substantial portionof the propylene and lesser percentages of propane and carbon dioxide.The nitrogen, oxygen and the remainder of the carbon oxides and propaneare withdrawn from the system, preferably combusted, and vented. It iscontemplated herein to recover oxygen from the vent stream and recycleit to the reactor feed to enhance the operation of the system.

Utilizing a system, as shown in FIG. 3, for the production ofacrylonitrile utilizing oxygen-enriched air containing approximately 60percent by volume of oxygen as a reactor feed and a mixture of nitrogen,carbon dioxide and propane as the gaseous flame suppressor, withnitrogen comprising a major portion thereof, the flow rates in molepercent at critical points in the system were determined and arepresented in Table I. The feed was obtained by mixing equal parts of airand pure oxygen. Flow rates were also determined using air as the feedto the ammoxidation reactor and are also reported in Table I. Thecompositions are expressed in mole percent and based on 100 moles ofacrylonitrile produced. The data expressed in Table I representsoperation of the system under conditions such that 60 percent and 80percent, respectively, of the propylene feed to the ammoxidation reactoris coverted to products for each feed. In Table I, Point A is the feedinto the ammoxidation reactor, Point B is effluent therefrom, Point C isthe feed to the compressor, Point D is the recycle stream from the PSAseparator, and Point E is the vent stream from the PSA separator.

                  TABLE I                                                         ______________________________________                                        Component  A        B      C      D    E                                      ______________________________________                                        60 Percent Conversion - Equal parts pure oxygen and air                       Propylene  15.3     5.8    9.2    30.4 0.3                                    Propane    8.2      7.8    12.4   40.8 0.5                                    Oxygen     21.1     4.0    6.4    0.4  8.9                                    CO         --       0.7    1.1    0.1  1.5                                    CO.sub.2   4.8      6.5    10.3   24.2 4.4                                    Acrylonitrile                                                                            --       7.2    --     --   --                                     Acrolein   --       0.1    --     --   --                                     Acetonitrile                                                                             --       0.1    --     --   --                                     HCN        --       1.3    --     --   --                                     Water      --       27.3   --     --   --                                     Ammonia    10.3     1.0    --     --   --                                     Nitrogen   40.3     38.2   60.6   4.1  84.4                                   80 Percent Conversion - Equal parts pure oxygen and air                       Propylene  11.7     2.2    3.6    13.6 0.1                                    Propane    8.4      7.9    12.8   48.3 0.5                                    Oxygen     21.9     4.0    6.5    0.5  8.5                                    CO         --       0.8    1.3    --   1.7                                    CO.sub.2   5.5      7.5    12.1   33.7 4.9                                    Acrylonitrile                                                                            --       7.2    --     --   --                                     Acrolein   --       0.1    --     --   --                                     Acetonitrile                                                                             --       0.1    --     --   --                                     HCN        --       1.6    --     --   --                                     Water      --       28.0   --     --   --                                     Ammonia    10.5     1.0    --     --   --                                     Nitrogen   42.0     39.6   63.7   4.9  84.3                                   60 Percent Conversion - Air                                                   Propylene  10.3     4.0    5.4    30.8 0.2                                    Propane    2.6      2.5    3.4    19.1 0.2                                    Oxygen     15.6     4.0    5.4    1.6  6.2                                    CO         --       0.5    0.6    0.2  0.7                                    CO.sub.2   3.3      4.5    6.1    25.0 2.2                                    Acrylonitrile                                                                            --       5.0    --     --   --                                     Acrolein   --       --     --     --   --                                     Acetonitrile                                                                             --       0.1    --     --   --                                     HCN        --       1.0    --     --   --                                     Water      --       18.8   --     --   --                                     Ammonia    7.3      1.0    --     --   --                                     Nitrogen   60.9     58.6   79.1   23.4 90.5                                   80 Percent Conversion - Air                                                   Propylene  7.7      1.5    2.0    10.5 0.2                                    Propane    1.3      1.3    1.7    9.2  0.2                                    Oxygen     15.8     4.0    5.4    3.2  5.8                                    CO         0.1      0.6    0.8    0.5  0.9                                    CO.sub.2   3.7      5.0    6.8    27.8 2.5                                    Acrylonitrile                                                                            --       4.8    --     --   --                                     Acrolein   --       --     --     --   --                                     Acetonitrile                                                                             --       0.1    --     --   --                                     HCN        --       1.1    --     --   --                                     Water      --       18.8   --     --   --                                     Ammonia    7.2      1.0    --     --   --                                     Nitrogen   64.2     61.8   83.3   48.8 90.4                                   ______________________________________                                    

The data in Table I represents a typical commercial propylene sourcewhich comprised about 95 percent propylene with the remainder beingpropane.

In FIG. 4, there is illustrated a system utilizing a pure oxygen feed tothe reactor wherein the gaseous flame suppressor is comprised of propaneand carbon dioxide. On a volume-to-volume basis, propane is the mosteffective component of the subject gaseous flame suppressor mixtures. Inthe system shown in FIG. 4, the gas phase effluent from the quench toweris introduced into a separator, preferably a PSA unit, to separate thepropylene and at least a portion of the propane for recycle to thecombined reactor feed. As in the system shown in FIG. 3, the volumepercent of propane in the reactor feed is determined so as to produce aconcentration in the gaseous feed from the quench tower to thecompressor which, in combination with the carbon dioxide present, willminimize flammability potential.

The gas phase effluent from the quench tower preferably contains, on avolume basis, from about 10 to 40 percent of propylene, from about 10 to25 percent of oxygen, from about 10 to 45 percent of carbon dioxide,from about 2 to 5 percent of carbon monoxide and from about 10 to 50,preferably about 40 percent, of propane. More preferably, the combinedvolume percent of propylene, propane and carbon monoxide in the quenchtower effluent is from about 35 to 70 percent by volume.

The amount of propane in the recycle stream can be the predeterminedminimum amount previously given, but typically will be lower tocompensate for the amount of propane present in the propylene supply asan impurity. The selective separation capacity of the PSA unit providesnot only recycle of a substantial portion of the unreacted propylene tothe reactor, but for a relatively constant level of propane in thesystem.

Utilizing a system as shown in FIG. 4 for the production ofacrylonitrile utilizing a gaseous flame suppressor mixture with propaneas major component, the flow rates in mole percent at critical points inthe system were determined and are presented in Table II. Thecompositions are expressed in mole percent and based on 100 moles ofacrylonitrile produced. The data expressed in Table II representsoperation of the system under conditions such that 60 percent and 80percent, respectively, of the propylene feed to the ammoxidation reactoris converted to products. In Table II, the various parts are the same asin Table I.

                  TABLE II                                                        ______________________________________                                        Component  A        B      C      D    E                                      ______________________________________                                        60 Percent Conversion                                                         Propylene  27.8     9.3    28.0   39.4 2.3                                    Propane    8.8      7.4    22.3   28.8 7.4                                    Oxygen     36.7     5.0    15.0   --   49.1                                   CO         --       1.0    3.2    --   10.3                                   CO.sub.2   8.8      10.5   31.5   31.8 31.0                                   Acrylonitrile                                                                            --       11.6   --     --   --                                     Acrolein   --       0.1    --     --   --                                     Acetonitrile                                                                             --       0.2    --     --   --                                     HCN        --       2.3    --     --   --                                     Water      --       51.5   --     --   --                                     Ammonia    18.0     1.0    --     --   --                                     80 Percent Conversion                                                         Propylene  28.5     9.6    28.7   40.3 2.4                                    Propane    9.1      7.6    22.8   29.6 7.5                                    Oxygen     37.6     5.1    15.5   --   50.7                                   CO         --       1.3    3.8    --   12.4                                   CO.sub.2   9.9      11.9   35.7   36.0 35.1                                   Acrylonitrile                                                                            --       11.4   --     --   --                                     Acrolein   --       0.1    --     --   --                                     Acetonitrile                                                                             --       0.2    --     --   --                                     HCN        --       2.5    --     --   --                                     Water      --       52.7   --     --   --                                     Ammonia    24.7     1.0    --     --   --                                     ______________________________________                                    

The data presented in Table II represents a typical commercial propylenesource which comprised about 95 percent propylene with the remainderbeing propane. The run was repeated utilizing high purity propylenewhich is about 98 percent pure. The results are reported in Table III.

                  TABLE III                                                       ______________________________________                                        Component  A        B      C      D    E                                      ______________________________________                                        60 Percent Conversion                                                         Propylene  29.5     9.8    32.6   47.6 2.5                                    Propane    3.8      3.1    10.4   14.0 3.2                                    Oxygen     38.5     5.0    16.6   --   50.1                                   CO         --       1.1    3.7    --   11.1                                   CO.sub.2   9.3      11.0   36.7   38.4 33.2                                   Acrylonitrile                                                                            --       12.2   --     --   --                                     Acrolein   --       0.1    --     --   --                                     Acetonitrile                                                                             --       0.2    --     --   --                                     HCN        --       2.4    --     --   --                                     Water      --       54.0   --     --   --                                     Ammonia    25.0     1.0    --     --   --                                     80 Percent Conversion                                                         Propylene  30.2     10.1   33.4   48.8 2.5                                    Propane    3.8      3.2    10.6   14.3 3.2                                    Oxygen     39.5     5.1    17.2   --   51.8                                   CO         --       1.3    4.4    --   13.3                                   CO.sub.2   10.5     12.5   41.6   43.6 37.6                                   Acrylonitrile                                                                            --       11.9   --     --   --                                     Acrolein   --       0.1    --     --   --                                     Acetonitrile                                                                             --       0.2    --     --   --                                     HCN        --       2.7    --     --   --                                     Water      --       55.3   --     --   --                                     Ammonia    25.0     1.0    --     --   --                                     ______________________________________                                    

In the embodiment of the present invention illustrated in FIG. 5, a pureoxygen feed to the ammoxidation reactor it utilized with a gaseous flamesuppressor mixture containing carbon dioxide as the major component. Inthis instance, the percent of carbon dioxide in the feed to theselective separator is from about 25 to 70, preferably from about 35 to65, percent by volume. The selective separator may be a PSA unit or aconventional liquid separator operating on the well-known Benfieldprocess wherein carbon dioxide is removed from the effluent bydissolution n an absorbent solution, such as potassium carbonate. Ineither instance, however, although the separator may be run at ambientpressure, it is preferred that the feed to the separator be introducedunder pressure utilizing, e. g. a compressor, as shown in FIG. 5. When aliquid separator is utilized, the resultant solution is regenerated torelease carbon dioxide. Wherein the separator is a PSA unit, theadsorbent is suitably a zeolite molecular sieve such as 4A molecularsieve or a silic gel, preferably the latter.

Since carbon dioxide is produced in the ammoxidation reactor as anundesirable by-product, it is preferred to have only a small quantitythereof recycled in the reactor feed. In addition, it is advantageous tominimize the carbon dioxide content in the reactor to prevent anundesirable pressure build-up which would decrease the efficiency of theprocess. Typically, the feed to the ammoxidation reactor comprisespropylene, oxygen, ammonia and about 5 percent by volume or less ofcarbon dioxide.

In the embodiment shown in FIG. 5, carbon dioxide separated in theseparator is at least partially recycled upstream of the compressor toassure that the concentration entering the compressor is from about 25to 70 percent by volume. Since it is not necessary to have a highinitial concentration of carbon dioxide in the reactor, the recyclethereof is within the system to achieve the desired concentrationthereof in the gaseous flame suppressor. The recycle may reenter thesystem either upstream or downstream of the quench tower or at bothlocations as shown in FIG. 5. In the event that the concentration ofcarbon dioxide in the stream entering the compressor becomes too great,a portion of the recycle stream is vented from the system as shown inFIG. 5. The operating pressure of the selective separator is typicallyfrom about 10 to 50, preferably from about 20 to 40, psig. The recycleof carbon dioxide as shown in FIG. 5 is carried out even in thoseinstances where the selective separator is operated at close to ambientpressure and there is no compressor in the system since it providesbetter control over the amount of carbon dioxide in the system.

The gas mixture exiting the selective separator comprises carbondioxide, carbon monoxide, oxygen and unreacted propylene. A Benfieldprocess selective separator will generally reduce the carbon dioxidecontent of the effluent gas mixture to about 5 percent by volume. A PSAunit selective separator will generally reduce the carbon dioxidecontent of the effluent to about 10 percent by volume. Unreactedreactant hydrocarbon is also present in the system to a degree varyingwith the reactor conversion as disclosed above and is substantially allrecycled from the selective separator. Fresh reactant hydrocarbon isadded to this mixture to increase the pressure thereof, if necessary,and it is recycled to the ammoxidation reactor where it is combined withincoming oxygen, ammonia and additional reactant hydrocarbon, e.g.propylene, to produce a final carbon dioxide concentration of about 1percent or less by volume.

Table IV gives an indication of the operating parameters of a system forthe synthesis of acrylonitrile as shown in FIG. 5 wherein carbon dioxideis a major component of the gaseous flame suppressor and the selectiveseparator is a conventional liquid separator. As in the previous Tables,the data in Table IV are expressed in mole percent based on 100 moles ofacrylonitrile produced. The data in Table IV are also for 60 and 80percent propylene conversion and are for the following points in theprocess: A is feed into the ammoxidation reactor; B is effluenttherefrom; C is effluent from the quench tower prior to the addition ofrecycled carbon dioxide thereto; D is the feed to the compressor; E isthe recycle stream from the separator to the reactor; and F is thecarbon dioxide recycle from the selective separator.

                  TABLE IV                                                        ______________________________________                                        Component A       B       C     D     E     F                                 ______________________________________                                        60 Percent Conversion                                                         Propylene 33.3    11.9    52.4  33.4  65.4  0.8                               Propane   1.5     1.3     5.7   3.6   1.9   0.1                               Oxygen    42.8    5.0     21.9  14.0  25.9  0.3                               CO        1.1     2.0     4.7   3.0   5.6   0.1                               CO.sub.2  0.2     3.7     15.3  45.9  1.2   98.7                              Acrylonitrile                                                                           --      15.4    --    --    --    --                                Acrolein  --      0.1     --    --    --    --                                Acetonitrile                                                                            --      0.2     --    --    --    --                                HCN       --      2.4     --    --    --    --                                Water     --      57.0    --    --    --    --                                Ammonia   21.1    1.0     --    --    --    --                                80 Percent Conversion                                                         Propylene 27.0    4.8     28.4  13.5  28.2  0.2                               Propane   1.5     1.4     8.1   3.8   2.1   0.1                               Oxygen    47.2    5.0     29.5  14.0  27.8  0.2                               CO        1.3     2.2     6.8   3.2   6.4   0.1                               CO.sub.2  0.3     4.9     27.2  65.5  1.8   99.4                              Acrylonitrile                                                                           --      16.1    --    --    --    --                                Acrolein  --      0.1     --    --    --    --                                Acetonitrile                                                                            --      0.1     --    --    --    --                                HCN       --      2.9     --    --    --    --                                Water     --      61.5    --    --    --    --                                Ammonia   22.7    1.0     --    --    --    --                                ______________________________________                                    

In Table V, the system as shown in FIG. 5, but utilizing a PSA unit as aseparator. In Table IV, points A through D are as in Table IV, Point Eis the recycle gaseous stream from the PSA unit, and point F is the CO₂recycle stream from the PSA unit.

                  TABLE V                                                         ______________________________________                                        Component A       B       C     D     E     F                                 ______________________________________                                        60 Percent Conversion                                                         Propylene 32.9    11.8    52.2  33.1  63.5  7.4                               Propane   1.4     1.3     5.7   3.6   1.8   0.8                               Oxygen    43.0    5.0     22.1  14.0  25.4  3.1                               CO        1.0     2.0     4.7   3.0   5.4   0.7                               CO.sub.2  0.8     4.1     15.3  46.3  3.9   88.0                              Acrylonitrile                                                                           --      15.3    --    --    --    --                                Acrolein  --      0.1     --    --    --    --                                Acetonitrile                                                                            --      0.1     --    --    --    --                                HCN       --      2.4     --    --    --    --                                Water     --      56.9    --    --    --    --                                Ammonia   20.9    1.0     --    --    --    --                                80 Percent Conversion                                                         Propylene 26.5    4.7     28.3  13.2  39.7  2.2                               Propane   1.5     1.3     8.0   3.8   3.0   0.6                               Oxygen    47.5    5.0     29.8  14.0  39.7  2.4                               CO        1.1     2.2     6.8   3.2   9.0   0.5                               CO.sub.2  1.1     5.5     27.1  65.8  8.6   94.3                              Acrylonitrile                                                                           --      15.9    --    --    --    --                                Acrolein  --      0.1     --    --    --    --                                Acetonitrile                                                                            --      0.1     --    --    --    --                                HCN       --      2.8     --    --    --    --                                Water     --      61.4    --    --    --    --                                Ammonia   22.3    1.0     --    --    --    --                                ______________________________________                                    

It will be appreciated that, when carbon dioxide or a substantiallyunreactive hydrocarbon is the major component of the gaseous flamesuppressor in the process of the present invention, it will be necessaryinitially to add a sufficient amount thereof to the feed to theammoxidation or oxidation reactor to establish the desired concentrationthereof, i.e. to prime system. This is, however, not necessary utilizingoxygen-enriched air as the reactor feed wherein nitrogen is the majorcomponent of the gaseous flame suppressor. It will likewise beappreciated that it is within the scope of the present invention toutilize conventional equipment to monitor and automatically regulate theflow of gases within the system so that it can be fully automated to runcontinuously in an efficient manner.

The process of this invention is advantageous in its simplicity, ease ofoperation, low capital and operating costs as well as providing asubstantially reduced flammability potential. In the process illustratedin FIG. 3, it is possible to utilize a less expensive grade of propylenewhich contains appreciable quantities of propane as an impurity.Likewise, the use of air or oxygen-enriched air as a starting materialwherein nitrogen is the major component of the gaseous flame suppressor,provides further economic savings. The subject process can be run at arelatively low conversion of the feed hydrocarbon to the desired productto achieve substantially improved selectivity. Selectivity is the amountof olefin converted to desired product divided by the total of olefinconverted. It will be appreciated that a system that runs atcomparatively low conversion and achieves enhanced selectivity to adesired product utilizing a less expensive grade of feed materials ishighly advantageous.

We claim:
 1. A cyclic process for the production of alpha, beta olefinically unsaturated nitriles comprising:(a) reacting a feed comprising an olefin, an oxygen-containing gas and ammonia in the vapor phase in the presence of an ammoxidation catalyst in a reactor vessel under condition which produce an effluent containing nitrile at low feed conversion and high product selectivity; (b) quenching the effluent with a liquid to form a quenched liquid phase containing said nitrile and a gaseous phase containing gaseous products including unreacted olefin; (c) recovering said nitrile from the liquid phase; (d) introducing the gaseous phase as a pressurized feed into a selective separator to thereby remove a substantial portion of the unreacted reactant olefin; and recycling said unreacted reactant olefin to said reactor vessel, wherein the process is conducted in the presence of a gaseous flame suppressor, the process being controlled so that flame suppression will be maximized at the point in the process where the pressure is increased, the amount of gaseous flame suppressor being such that the gaseous phase formed in step (b) contains from about 30 to about 95 percent by volume thereof.
 2. A process in accordance with claim 1, wherein the oxygen-containing gas is selected from the group consisting of pure oxygen, air and a gas enriched in oxygen relative to air.
 3. A process in accordance with claim 2, wherein the oxygen-containing gas is air of a gas enriched in oxygen relative to air and said gaseous flame suppressor comprises a substantially unreactive hydrocarbon containing 1 to 5 carbon atoms, carbon dioxide and nitrogen.
 4. A process in accordance with claim 3, wherein said substantially unreacted hydrocarbon is a saturated hydrocarbon.
 5. A process in accordance with claim 2, wherein the oxygen-containing gas is pure oxygen and said gaseous flame suppressor comprises carbon dioxide and a substantially unreactive hydrocarbon containing 1 to 5 carbon atoms.
 6. A process in accordance with claim 5, wherein said substantially unreacted hydrocarbon is a saturated hydrocarbon.
 7. A process in accordance with claim 1, wherein said selective separator is a pressure swing adsorption unit.
 8. A process in accordance with claim 7, wherein said pressure swing adsorption unit contains an adsorbent consisting of a silica gel or a zeolite molecular sieve.
 9. A process in accordance with claim 5, wherein carbon dioxide is a major component of said gaseous flame suppressor and said selective separator is a liquid separator wherein carbon dioxide is removed from the gaseous phase by dissolution into an absorbent solution.
 10. A process in accordance with claim 9, wherein the absorbent solution is an aqueous potassium carbonate solution.
 11. A process in accordance with claim 10, wherein at least a portion of the carbon dioxide obtained in said separator is recycled to the effluent formed in step (a), the gas phase formed in step (b) or both of said effluent and said gas phase.
 12. A process in accordance with claim 1, wherein the nitrile is acrylonitrile, the reactant hydrocarbon is propylene and the gaseous flame suppressor comprises carbon dioxide, propane and, where said oxygen-containing gas is air or a gas enriched in oxygen relative to air, nitrogen.
 13. A process in accordance with claim 5, wherein the nitrile is acrylonitrile, the reactant hydrocarbon is propylene and the flame suppressor is carbon dioxide and propane, wherein the gas phase formed in step (b) comprises unreacted propylene, oxygen, carbon dioxide, carbon monoxide and from about 10 to 50 volume percent of propane, said selective separator is a pressure swing. adsorption unit and the gaseous fraction formed therein contains propane and a substantial portion of the unreacted propylene. 